This invention is concerned with a process for producing thermosetting coating compositions of oil-free alkyd resins in combination with aminoplast resins, which can optionally be rendered water soluble through reaction with acids.
Oil-free alkyd resins, and more particularly saturated polyester resins, are excellent binders for coating compositions which combine superior hardness with outstanding flexibility and which are particularly resistant to solvents and weathering conditions. A combination of hardness and flexibility are the exigencies for paints applied by the coil coating process.
The known oil-free alkyds are linear or branched polyesters of diols and triols and/or tetrols, the hydroxy groups of which are terminal or distributed randomly in the polymer chain and which are combined with aminoplast resins as cross-linking agents. These polyesters in many cases have the disadvantage that they require very high stoving temperatures in order to attain the optimum properties. A reduction of the stoving temperatures can only be effected within a limited range by using acid catalysts which, however, usually cause a reduction in overall quality. A further disadvantage of these polyesters is their poor compatibility with butylated aminoplast resins, requiring judicious selection of special grades of materials to permit their use in combination. Another disadvantage is the poor dilutability of the resin blends with aromatic hydrocarbon solvents, unless greater amounts of alcohols, esters, and ketones are co-employed.